Thin-film rechargeable batteries have numerous applications in the field of microelectronics. For example, thin-film batteries may provide active or standby power for microelectronic devices and circuits. Active power source applications of the thin-film battery include, for example, implantable medical devices, remote sensors, wireless sensors, semiconductor diagnostic wafers, automobile tire sensors, miniature transmitters, active radio frequency identification (RFID) tags, smart cards, and MEMS devices. Standby power source applications of thin-film batteries include non-volatile CMOS-SRAM memory products such as memory ships for computers, sensors, and passive RFID tags.
In a thin-film battery, a chemical reaction takes place between an anode and cathode by interaction of the anode and cathode through an electrolyte. The attractiveness of thin-film batteries over conventional batteries is that the electrolyte is a solid or non-flowable material rather than a liquid. Liquid electrolytes pose leakage problems and are often highly corrosive. Of the solid electrolytes, thin-film batteries typically employ glassy ceramic electrolytes. Solid electrolytes are desirable in cells or batteries where liquid electrolytes may be undesirable, such as in implantable medical devices. Preferred solid electrolytes include materials that are amorphous solids with high melting temperatures (greater than about 900° C.), electrically insulative and ionically conductive.
One of the challenges for thin film battery manufacturers is to provide a thin film battery that will have an extended life when exposed to an oxygen-containing atmosphere. An extended life is particularly difficult to obtain with thin film batteries containing anode materials which are highly reactive with oxygen and/or water or water vapor. Various barrier materials have been applied to thin film batteries to reduce the reactivity of the anode materials toward oxygen and/or water or water vapor. However, such barrier materials have met with limited success.
For example thin film batteries must be sealed or packaged in barrier materials in order to be able to operate in an air environment for a practical length of time. A suitable package must limit the permeation of oxygen and water vapor to such a small level as to allow at least 80% of the battery's capacity to be available after months to years of storage and/or operation. Thin film batteries can be stored in dry environments in which the relative humidity is sufficiently low that water vapor permeation is not a life-limiting factor. However, exposure to air reduces battery life to a few days if oxygen permeation is not restricted to a sufficiently low level by a suitable barrier package. In applications such as automobile tire sensors wherein wireless sensors are imbedded in the sidewalls of the tire, thin film batteries also must be protected from hydrostatic pressure.
A thin film encapsulation process is the preferred method of sealing, because the encapsulation layers may be deposited using the same equipment employed in making the batteries. However, silicon, tin, and silicon-tin alloy anodes of thin film lithium-ion batteries may expand uniaxially along the orthogonal direction to the film by over 250% during a charge step. Such expansion strains the protective encapsulation material to the point of fracture allowing oxygen and water vapor to rapidly reach the anode. While a polymer film may be able to accommodate the strain imposed by an expanding anode, a polymer film alone does not provide a sufficient barrier to oxygen and water vapor.
As advances are made in microelectronic devices, new uses for thin-film batteries continue to emerge. Along with the new uses, there is a need for high performance thin-film batteries having improved life. In particular, there is a need for rechargeable thin film batteries that have a life approaching at least five years or longer. Accordingly, there continues to be a need for improved hermetic seals for thin film batteries that enable use of such long life batteries in new applications. There is also a need for batteries that are able to withstand hydrostatic pressures above atmospheric pressure.
With regard to the above, there is provided in one embodiment a method for improving the useful life of a thin film lithium-ion battery containing a solid electrolyte and an anode that expands on charging and long life batteries made by the method. The method includes providing a hermetic barrier package for the thin film battery that includes an anode expansion absorbing structure.
Another embodiment of the disclosure provides a thin film lithium-ion battery having, a cathode, a solid lithium-ion conducting electrolyte, an anode selected from the group consisting of silicon, tin, and silicon-tin alloys. The battery includes a hermetic seal, wherein the hermetic seal has an anode expansion absorbing structure.
Yet another embodiment of the disclosure provides a method of making multiple long-life thin film battery cells on a single substrate. The method includes the step of depositing battery layers including cathodes, electrolytes, and anodes through appropriate masks onto the substrate. A hermetic seal is constructed for each of the cells. The hermetic seal has an anode expansion absorbing structure. The open circuit voltage and resistance of each of the cells is determined using a wafer prober in conjunction with a programmable electrometer to identify rejected cells. Rejected cells are ink marked, and the substrate is diced to provide a plurality of thin film batteries.
An advantage of the disclosed embodiments is that improved hermetic seals for thin film batteries having anodes that greatly expand on charging may be provided. While conventional thin film batteries containing lithium anodes and anodes that do not greatly expand on charging may use conventional hermetic seals, thin film batteries that have anodes that expand over about 200 percent of their height may benefit from the improved hermetic seals and sealing methods provided herein.